As one conventional device for detecting a physical quantity, as disclosed in, for example, JP,A 3-233168, an air flowmeter with a resistance heater has a structure in which a housing section containing an electronic circuit is formed integrally with an auxiliary air passage section, and the housing section and the auxiliary air passage section are arranged side by side on one plane. The air flowmeter is mounted to a body, in which an intake air passage of an internal combustion engine is formed, such that a flow rate detecting unit disposed in the auxiliary air passage section is located in the intake air passage.